Introduce system shortcut key unit tests

Provide an enviroment that could verify if a key or a combination keys
could be processed properly by the policy and execute the shortcut
behavior.

This includes these tests to for phone device:

CombinationKeyTests - test cases for combination keys
                        - power + volume down
                        - power + volume up
                        - volume down + volume up

PowerKeyGestureTests - test cases for power key gesture
                        - short press
                        - double presses
                        - long press

Bug: 193278327
Test: atest PowerKeyGestureTests CombinationKeyTests
Change-Id: I97424a7acb4a0d02c3bf29343c6fc552da0f7d65